For example, Japanese patents Nos. 3,219,955, 3,108,820 and 3,090,547 have disclosed collision airflow pulverizers which pulverize a material using jet stream to produce a particulate material having an average particle diameter on the order of microns. The pulverizers include a compressed gas supplying nozzle, an accelerating tube, a pulverizing chamber including a collision member therein, and a classifier. The accelerating tube has an entrance, from which a compressed gas is fed into the accelerating tube, an inlet from which a raw material to be pulverized is supplied, and an exit from which a mixture of the compressed gas and the supplied raw material is ejected. The entrance of the accelerating tube is connected with the compressed gas supplying nozzle, and the exit thereof is connected with the pulverizing chamber in such a manner that the exit faces the collision member.
In the above-mentioned pulverizers, a raw material is pulverized as follows. Initially, a compressed gas supplied to the compressed gas supplying nozzle is further compressed therein while accelerated to a subsonic speed. The compressed gas thus accelerated is supplied to the accelerated tube, and the accelerated tube accelerates the compressed gas while controlling expansion of the gas. On the way of the acceleration operation, a raw material to be pulverized is supplied from the inlet to be mixed with the compressed gas. The solid-gas mixture of the compressed gas and the raw material is further accelerated in the accelerating tube, and then ejected from the exit of the accelerating tube. The raw material in the ejected mixture is collided with the collision member, resulting in pulverization of the raw material. The pulverized raw material (i.e., a particulate material) is collected by the classifier, and particles having particle diameters in the desired particle diameter range are collected while particles having particle diameters greater than the desired particle diameter range are fed to the inlet of the accelerating tube to be pulverized again.
However, in the pulverizers mentioned above, in which the inlet is provided on a portion of the accelerating tube, rapid density change is caused in the vicinity of the inlet, and therefore a problem in that a shock wave such as a diamond shock wave is generated tends to be caused. When such a shock wave is generated, the velocity of the solid-gas mixture of the compressed gas and the raw material is decreased, and therefore it becomes difficult to eject the solid-gas mixture at the desired velocity. As a result, the collision energy of the raw material collided with the collision member is seriously decreased, and it becomes difficult to produce a particulate material having the desired particle diameter by one collision pulverization operation, resulting in deterioration of the pulverization efficiency of the pulverizers.
A conventional pulverizer will be described in detail by reference to FIG. 6. FIG. 6 illustrates a conventional pulverizer 100a in which an inlet 9b is provided on a middle portion of an accelerating tube 9. The pulverizer 100a includes a compressed gas supplying nozzle 8, the accelerating tube 9, a raw material supplier 1 to supply a raw material to be pulverized, a pulverizing chamber 10 in which a collision member 11 is provided, a cyclone 12 to separate pulverized particles, and a hopper 13 to collect the pulverized particles (i.e., a product). Numerals 5 and 6 respectively denote a pressure adjusting valve and a pressure controller, which serve as a pressure controller, and numeral 7 denotes a gas flow pipe.
In the pulverizer 100a, the raw material to be pulverized is supplied to the accelerated tube 9 from the raw material suppler 1 through the inlet 9b. Therefore, rapid density change is caused in the vicinity of the inlet 9b, thereby often generating a shock wave such as a diamond shock wave. When such a shock wave is generated, the velocity of the solid-gas mixture of the compressed gas and the raw material in the accelerating tube 9 is decreased, and therefore it becomes difficult to eject the mixture at the desired velocity. As a result, the collision energy of the raw material collided with the collision member is seriously decreased, and it becomes difficult to produce a particulate material having the desired particle diameter by one collision pulverization operation, resulting in deterioration of the pulverization efficiency of the pulverizers.
In addition, generation of a shock wave not only decreases the velocity of the solid-gas mixture of the compressed gas and the raw material in the accelerating tube 9, but also forms airflow flowing toward a lower side (bottom) of the accelerating tube 9. Therefore, the solid-gas mixture is mainly fed to a lower side of the collision member, and the particles of the raw material are collided with substantially the same portion on the lower side of the collision member 11. As a result, the portion of the collision member is seriously abraded, and therefore the collision member 9 has to be frequently replaced with a new collision member, resulting in decrease of the maintenance operation cycle and increase of the maintenance costs.
In attempting to prevent occurrence of the problem, a published unexamined Japanese patent application No. 2010-284634 (hereinafter JP2010-284634A) discloses a pulverizer which includes a solid-gas supplying nozzle, and an accelerating tube to eject a raw material to be pulverized toward a collision member. The pulverizer further includes a solid-gas mixer to mix a compressed gas and the raw material while supplying the solid-gas mixture to the solid-gas supplying nozzle. Since the solid-gas mixture of the compressed gas and the material prepared by the solid-gas mixer is supplied to the accelerating tube through the solid-gas supplying nozzle, it is not necessary to form an inlet, from which the raw material is supplied to the accelerating tube, on the accelerating tube, and therefore generation of a shock wave can be prevented, thereby preventing decrease of the velocity of the solid-gas mixture of the compressed gas and the raw material, resulting in ejection of the solid-gas mixture from an ejection opening of the accelerating tube at a desired velocity. Therefore, a particulate material (product) having the desired particle diameter can be prepared by one collision pulverization operation, namely a particulate material can be prepared without causing the pulverization efficiency deterioration problem caused by a shock wave.
However, the particle diameter of particles of the raw material, which are supplied to the solid-gas mixer, varies. The particles of the raw material in the solid-gas mixture supplied to the solid-gas supplying nozzle are fed along a path corresponding to the flowing direction of the solid-gas mixture regardless of the particle diameters of the particles, and then collided with the collision member. Therefore, particles having a relatively small particle diameter tend to be excessively pulverized, and the pulverized particles tend to have a smaller particle diameter than the targeted particle diameter. In contrast, particles having a relatively large particle diameter tend to be insufficiently pulverized, and the pulverized particles tend to have a larger particle diameter than the targeted particle diameter. Therefore the pulverized particles have to be returned to the solid-gas mixer, resulting in deterioration of the yield of the product.
For these reasons, the inventors recognized that there is a need for a pulverizer which can pulverize a raw material with a high pulverization efficiency without generating a shock wave in an accelerating tube, resulting in production of a pulverized material at a high yield.